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Every Mechanism

Home How a Steam Locomotive Works

Fire, water and steam

How a steam locomotive works

A coal fire boils water into high-pressure steam. The steam pushes two pistons back and forth, and strong rods turn that push into the turning of the wheels. Then the used steam blasts up the chimney and makes the fire burn hotter.

Slide the valve yourself
14bar
steam pressure
1.52m
driving wheels
4
chuffs every turn
2-6-0
wheel arrangement
A crimson steam locomotive and its tender at dusk, cut open along its middle: the glowing fire in the firebox, the fire tubes in the blue boiler water, and the left cylinder with its piston, with steam rising from the chimney.
Cut open along its middle: the fire, the tubes in the boiler water, and the left cylinder.

Video

How a Steam Locomotive Works - Fire, Water and Steam

Watch on YouTube

3D model · cut open · 360°

Open the regulator

The engine is already cut open along its middle. Open the regulator and the steam drives the pistons, the rods and the valve gear. Move the reverser to change the cut-off or to run backwards. Drag to turn the view, scroll or pinch to zoom, and click a part to use it.

Try itSlide the regulator open. Steam from the boiler pushes the pistons, and you can hear each puff of used steam leave the chimney: four for every turn of the wheels.
Loading the 3D model…

Meet the parts

Sixteen parts, one engine

The engine cut open along its middle, and its left cylinder cut through the steam chest. Tap any part of the picture or pick a name below. The fire and the boiler are on top, the cylinders at the front, and the rods and valve gear along the wheels.

The locomotive cut open: the firebox with its fire at the back, the fire tubes in the boiler water, the dome, the smokebox and chimney at the front, the left cylinder cut open with its piston and valve, and the rods, wheels and valve gear below.

Hover or tap the picture. Red rings mark every part.

Parts diagram

From the fire to the wheels

The engine cut open, seen from its left side. The numbers follow the energy: 1 to 7 from the fire to the superheater, 8 to 16 where the steam does its work, 17 and 18 the exhaust, 19 to 22 the valve gear, and 23 to 26 the driver’s controls and the safety fittings.

The locomotive cut open from its left side, with twenty-six numbered parts from the firebox to the wheels.
    How it works, step by step

    Twenty-one steps from fire to motion

    The same steps as the video. Each picture is a moment from it; press Show me in 3D to set the model up for that step.

      The engine running at dusk, steam rising from its chimney.
      Step 1 of 21

      Fire and water into motion

        Demo · inside the cylinder

        Be the valve gear

        The left cylinder, cut open like the video’s drawing. Turn the wheel and watch the slide valve open one port to live steam and the other to the exhaust. Then take the valve yourself and try to keep the wheel turning.

        The valve slides over three ports in the floor of the steam chest. Uncover a steam port and live steam from the boiler rushes into that end of the cylinder. Under the valve’s hollow, the other end is joined to the exhaust port, so its used steam can escape. Move the valve the wrong way, or at the wrong moment, and the steam pushes against the piston instead of with it.

        • Orange: live steam at up to 14 bar.
        • Blue: used steam on its way to the chimney.
        –front end, bar
        –back end, bar
        –push on the piston
        –power strokes · wheel turns

        Demo · the reverser

        Cut-off: power or economy

        The reverser sets how far through each stroke the valve lets steam in. To start a heavy train, the driver lets it in for most of the stroke. At speed, the driver cuts it off early and lets the steam already inside expand on its own, like a spring pushing out.

        –steam let in, % of the stroke
        –average push on the piston
        –steam used per turn of the wheels
        –work from each kilogram of steam

        Demo · two sides

        Why a quarter turn?

        A piston at the very end of its stroke pushes straight through the axle, and the wheel cannot turn. So the engine has a cylinder on each side, with their cranks a quarter of a turn apart. Turn the wheel to any position and try to start.

        The chart shows how hard each side can turn the wheel at every position, with full steam in the cylinder. One cylinder alone has two dead spots each turn. With the second side a quarter turn ahead, one of them is always pushing hard, so the engine can start wherever it stopped.

        • Left side: the cylinder you can see.
        • Right side: a quarter turn ahead.
        –left side turning force
        –right side turning force
        –both together
        –can it start here?

        Demo · the chuff

        The blast fans the fire

        Every puff of used steam shoots up the chimney from the blast pipe and drags the smoke along with it. That sucks the hot gases through the tubes and draws fresh air up through the fire. Work the engine harder and it blasts harder, so the fire burns hotter and makes more steam.

        –chuffs a second
        –draught in the smokebox, mm of water
        –fire
        –steam made · used, tonnes an hour

        More to know

        Kinds, history and care

        Six kinds of steam locomotive

        Tender engineThe engine on this page. Its coal and water ride behind in a separate tender, so it can run a long way between stops.
        Tank engineCarries its water in tanks beside or over the boiler and its coal in a bunker behind the cab. Handy for short runs and shunting, and it runs as well backwards as forwards.
        ArticulatedGarratts and Mallets carry two sets of cylinders and driving wheels under one huge boiler, so a giant can still get round curves. Union Pacific’s Big Boys weighed about 540 tonnes with their tenders.
        CompoundUses its steam twice: first in a small high-pressure cylinder, then in a large low-pressure one, to get more work from every kilogram. Anatole Mallet first tried it on a railway in 1876.
        GearedUpright cylinders turn a shaft along the side, and gears drive every wheel. Slow but very strong on the steep, rough track of logging railways, like Ephraim Shay’s engines.
        FirelessNo fire at all: its tank is filled with hot water and steam from a factory’s boiler. Used where a spark would be dangerous, such as at explosives works.

        From pumping engines to the last steam trains

        1. Thomas Newcomen’s engine pumps water out of a coal mine near Dudley: the first practical steam engine.
        2. Richard Trevithick’s locomotive hauls ten tons of iron and seventy men along a tramroad in South Wales: the first railway journey by steam.
        3. Locomotion No. 1 opens the Stockton and Darlington Railway, the first public railway to use steam locomotives.
        4. Stephenson’s Rocket wins the Rainhill Trials with a boiler full of fire tubes and a blast pipe up its chimney.
        5. Egide Walschaerts, a Belgian railway foreman, invents the valve gear used on this engine.
        6. Henri Giffard patents the injector, which uses a jet of steam to push water into the boiler.
        7. The first locomotive with Wilhelm Schmidt’s superheater is built for the Prussian railways.
        8. Mallard reaches 126 mph (203 km/h), still the speed record for steam.
        9. British Railways runs its last steam passenger train, on 11 August.
        10. Tornado is finished: the first new main-line steam locomotive built in Britain since 1960.

        Driving and firing it safely

        • Keep the water up. The water in the gauge glass must always cover the top of the firebox. If it ever falls too low, a lead plug in the firebox roof melts and lets steam into the fire as a warning.
        • Open the drain cocks before moving off from cold. Water that has condensed in a cylinder can’t be squashed, and a piston hitting it can break a cylinder cover.
        • Open the regulator gently. Too much steam at once makes the driving wheels spin on the rails. Sand from the sandboxes helps them grip.
        • Fire little and often. Thin layers of coal spread across the grate keep the fire bright and the smoke light.
        • Wash out the boiler regularly. Minerals from hard water build up as scale, which insulates the tubes and wastes coal.
        • Sweep the tubes and empty the smokebox. Blocked tubes or a leaking smokebox door kill the draught, and the engine runs short of steam.

        Things worth knowing

        Water grows into steamA litre of water boils into about 1,700 litres of steam. Trapped inside the boiler, that is where the pressure comes from.
        Four chuffs a turnEach cylinder lets out its used steam twice a turn, once from each end, so a two-cylinder engine beats four times for every turn of its wheels.
        Fastest of allMallard reached 126 mph on 3 July 1938. Its 2.03 m driving wheels were turning almost nine times a second.
        Steam that pushes waterThe injector uses a jet of the boiler’s own steam to force water back into that same boiler, against its full pressure, with no moving parts.
        Shovelling by the tonneBy about 1900 the biggest engines needed coal as fast as one fireman could shovel it, around two tonnes an hour. Mechanical stokers took over on the largest.
        Mostly heatA typical steam locomotive turned only about six per cent of the heat in its coal into pulling. Most of the rest went up the chimney.
        Quick check

        Four questions